This invention relates to a system for controlling the manner in which a motor is driven, and more particularly to a motor drive control system which is capable of preventing tool breakage in a numerically controlled machine tool.
Numerically controlled machine tools to subject a workpiece to a prescribed machining operation by employing a spindle motor to rotate a tool and a feed shaft motor to move the workpiece relative to the tool, thereby allowing the tool to machine the workpiece in the prescribed manner. There are occasions where tools break or where cutting blades become chipped in the numerically controlled machine tools of the above type. This can be attributed to either of the following two causes: (1) stopping the spindle motor with the blade edge biting into the workpiece while being subjected to a heavy load, and then withdrawing the tool from the workpiece; or (2) withdrawing the tool from the workpiece when the spindle motor is about to stop or after it has stopped, following a continuation in machining under an excessive load in an intermittent machining operation. More specifically, there are cases where machining is performed in an intermittent manner while the magnitude of the load is fluctuating. In such a machining operation the cutting edge of the tool bites into and machines the workpiece for a predetermined period of time, after which the spindle motor speed is reduced until rotation of the motor stops. At this time, however, the feed shaft motor is still operating and continues to move the workpiece relative to the tool. This can cause tool breakage since the spindle motor is operating at a lower speed or is already at rest with the tool still thrust against the workpiece. Even if the tool should manage to escape damage when the spindle motor is reduced in speed or stopped, such damage will be unavoidable when the tool is withdrawn from the workpiece after the spindle motor is stopped.